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RFC 5740

NACK-Oriented Reliable Multicast (NORM) Transport Protocol

Pages: 96
Proposed Standard
Obsoletes:  3940
Part 1 of 4 – Pages 1 to 18
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Network Working Group                                         B. Adamson
Request for Comments: 5740                     Naval Research Laboratory
Obsoletes: 3940                                               C. Bormann
Category: Standards Track                        Universitaet Bremen TZI
                                                              M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                           November 2009

       NACK-Oriented Reliable Multicast (NORM) Transport Protocol


This document describes the messages and procedures of the Negative- ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol. This protocol can provide end-to-end reliable transport of bulk data objects or streams over generic IP multicast routing and forwarding services. NORM uses a selective, negative acknowledgment mechanism for transport reliability and offers additional protocol mechanisms to allow for operation with minimal a priori coordination among senders and receivers. A congestion control scheme is specified to allow the NORM protocol to fairly share available network bandwidth with other transport protocols such as Transmission Control Protocol (TCP). It is capable of operating with both reciprocal multicast routing among senders and receivers and with asymmetric connectivity (possibly a unicast return path) between the senders and receivers. The protocol offers a number of features to allow different types of applications or possibly other higher-level transport protocols to utilize its service in different ways. The protocol leverages the use of FEC-based (forward error correction) repair and other IETF Reliable Multicast Transport (RMT) building blocks in its design. This document obsoletes RFC 3940. Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents

1. Introduction and Applicability . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.2. NORM Data Delivery Service Model . . . . . . . . . . . . . 5 1.3. NORM Scalability . . . . . . . . . . . . . . . . . . . . . 7 1.4. Environmental Requirements and Considerations . . . . . . 8 2. Architecture Definition . . . . . . . . . . . . . . . . . . . 8 2.1. Protocol Operation Overview . . . . . . . . . . . . . . . 10 2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . 12 2.3. Design Trade-Offs . . . . . . . . . . . . . . . . . . . . 12 3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 13 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 15 4.1. NORM Common Message Header and Extensions . . . . . . . . 15 4.2. Sender Messages . . . . . . . . . . . . . . . . . . . . . 18 4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . 18 4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . 28 4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . 29 4.3. Receiver Messages . . . . . . . . . . . . . . . . . . . . 47 4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . 47 4.3.2. NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53 4.4. General Purpose Messages . . . . . . . . . . . . . . . . . 55 4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . 55 5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 55 5.1. Sender Initialization and Transmission . . . . . . . . . . 57 5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . 58
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     5.2.  Receiver Initialization and Reception  . . . . . . . . . . 59
     5.3.  Receiver NACK Procedure  . . . . . . . . . . . . . . . . . 59
     5.4.  Sender NACK Processing and Response  . . . . . . . . . . . 62
       5.4.1.  Sender Repair State Aggregation  . . . . . . . . . . . 62
       5.4.2.  Sender FEC Repair Transmission Strategy  . . . . . . . 63
       5.4.3.  Sender NORM_CMD(SQUELCH) Generation  . . . . . . . . . 64
       5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
     5.5.  Additional Protocol Mechanisms . . . . . . . . . . . . . . 65
       5.5.1.  Group Round-Trip Time (GRTT) Collection  . . . . . . . 65
       5.5.2.  NORM Congestion Control Operation  . . . . . . . . . . 67
       5.5.3.  NORM Positive Acknowledgment Procedure . . . . . . . . 75
       5.5.4.  Group Size Estimate  . . . . . . . . . . . . . . . . . 77
   6.  Configurable Elements  . . . . . . . . . . . . . . . . . . . . 77
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 80
     7.1.  Baseline Secure NORM Operation . . . . . . . . . . . . . . 82
       7.1.1.  IPsec Approach . . . . . . . . . . . . . . . . . . . . 83
       7.1.2.  IPsec Requirements . . . . . . . . . . . . . . . . . . 85
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 86
     8.1.  Explicit IANA Assignment Guidelines  . . . . . . . . . . . 87
       8.1.1.  NORM Header Extension Types  . . . . . . . . . . . . . 87
       8.1.2.  NORM Stream Control Codes  . . . . . . . . . . . . . . 88
       8.1.3.  NORM_CMD Message Sub-Types . . . . . . . . . . . . . . 88
   9.  Suggested Use  . . . . . . . . . . . . . . . . . . . . . . . . 89
   10. Changes from RFC 3940  . . . . . . . . . . . . . . . . . . . . 90
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 91
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 91
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 91
     12.2. Informative References . . . . . . . . . . . . . . . . . . 92
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1. Introduction and Applicability

The Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol can provide reliable transport of data from one or more senders to a group of receivers over an IP multicast network. The primary design goals of NORM are to provide efficient, scalable, and robust bulk data (e.g., computer files, transmission of persistent data) transfer across possibly heterogeneous IP networks and topologies. The NORM protocol design provides support for distributed multicast session participation with minimal coordination among senders and receivers. NORM allows senders and receivers to dynamically join and leave multicast sessions at will with minimal overhead for control information and timing synchronization among participants. To accommodate this capability, NORM protocol message headers contain some common information allowing receivers to easily synchronize to senders throughout the lifetime of a reliable multicast session. NORM is self-adapting to a wide range of dynamic network conditions with little or no pre-configuration. The protocol is tolerant of inaccurate timing estimations or lossy conditions that can occur in many networks including mobile and wireless. The protocol can also converge and maintain efficient operation even in situations of heavy packet loss and large queuing or transmission delays. This document obsoletes the Experimental RFC 3940 specification. This document is a product of the IETF RMT working group and follows the guidelines provided in the Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents [RFC3269]. Statement of Intent This memo contains the definitions necessary to fully specify a Reliable Multicast Transport protocol in accordance with the criteria of IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols [RFC2357]. The NORM specification described in this document was previously published in the Experimental Category [RFC3940]. It was the stated intent of the RMT working group to re- submit this specifications as an IETF Proposed Standard in due course. This Proposed Standard specification is thus based on RFC 3940 and has been updated according to accumulated experience and growing protocol maturity since the publication of RFC 3940. Said experience applies both to this specification itself and to congestion control strategies related to the use of this specification. The differences between RFC 3940 and this document are listed in Section 10.
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1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. NORM Data Delivery Service Model

A NORM protocol instance (NormSession) is defined within the context of participants communicating connectionless (e.g., Internet Protocol (IP) or User Datagram Protocol (UDP)) packets over a network using pre-determined addresses and host port numbers. Generally, the participants exchange packets using an IP multicast group address, but unicast transport MAY also be established or applied as an adjunct to multicast delivery. In the case of multicast, the participating NormNodes will communicate using a common IP multicast group address and port number chosen via means outside the context of the given NormSession. Other existing IETF data format and protocol standards MAY be applied to describe and convey the necessary a priori information for a specific NormSession (e.g., Session Description Protocol (SDP) [RFC4566], Session Announcement Protocol (SAP) [RFC2974], etc.). The NORM protocol design is principally driven by the assumption of a single sender transmitting bulk data content to a group of receivers. However, the protocol MAY operate with multiple senders within the context of a single NormSession. In initial implementations of this protocol, it is anticipated that multiple senders will transmit independently of one another and receivers will maintain state as necessary for each sender. In future versions of NORM, it is possible some aspects of protocol operation (e.g., round-trip time collection) will provide for alternate modes allowing more efficient performance for applications requiring multiple senders. NORM provides for three types of bulk data content objects (NormObjects) to be reliably transported. These types include: 1. static computer memory data content (NORM_OBJECT_DATA type), 2. computer storage files (NORM_OBJECT_FILE type), and 3. non-finite streams of continuous data content (NORM_OBJECT_STREAM type). The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is simply to provide a hint to receivers in NormSessions serving multiple types of content as to what type of storage to allocate for received content (i.e., memory or file storage). Other than that
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   distinction, the two are identical, providing for reliable transport
   of finite (but potentially very large) units of content.  These
   static data and file services are anticipated to be useful for
   multicast-based cache applications with the ability to reliably
   provide transmission of large quantities of static data.  Other types
   of static data/file delivery services might make use of these
   transport object types, too.  The use of the NORM_OBJECT_STREAM type
   is at the application's discretion and could be used to carry static
   data or file content also.  The NORM reliable stream service opens up
   additional possibilities such as serialized reliable messaging or
   other unbounded, perhaps dynamically produced content.  The
   NORM_OBJECT_STREAM provides for reliable transport analogous to that
   of the Transmission Control Protocol (TCP), although NORM receivers
   will be able to begin receiving stream content at any point in time.
   The applicability of this feature will depend upon the application.

   The NORM protocol also allows for a small amount of out-of-band data
   (sent as NORM_INFO messages) to be attached to the data content
   objects transmitted by the sender.  This readily available out-of-
   band data allows multicast receivers to quickly and efficiently
   determine the nature of the corresponding data, file, or stream bulk
   content being transmitted.  This allows application-level control of
   the receiver node's participation in the current transport activity.
   This also allows the protocol to be flexible with minimal pre-
   coordination among senders and receivers.  The NORM_INFO content is
   atomic in that its size MUST fit into the payload portion of a single
   NORM message.

   NORM does NOT provide for global or application-level identification
   of data content within its message headers.  Note the NORM_INFO out-
   of-band data mechanism can be leveraged by the application for this
   purpose if desired, or identification can alternatively be embedded
   within the data content.  NORM does identify transmitted content
   (NormObjects) with transport identifiers that are applicable only
   while the sender is transmitting and/or repairing the given object.
   These transport data content identifiers (NormTransportIds) are
   assigned in a monotonically increasing fashion by each NORM sender
   during the course of a NormSession.  Participants, including senders,
   in NORM protocol sessions are also identified with unique identifiers
   (NormNodeIds).  Each sender maintains its NormTransportId assignments
   independently and thus individual NormObjects can be uniquely
   identified during transport by concatenation of the session-unique
   sender identifier (NormNodeId) and the assigned NormTransportId.  The
   NormTransportIds are assigned from a large, but fixed, numeric space
   in increasing order and will be reassigned during long-lived
   sessions.  The NORM protocol provides mechanisms so the sender
   application can terminate transmission of data content and inform the
   group of this in an efficient manner.  Other similar protocol control
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   mechanisms (e.g., session termination, receiver synchronization,
   etc.) are specified so reliable multicast application variants can
   realize different, complete bulk transfer communication models to
   meet their goals.

   To summarize, the NORM protocol provides reliable transport of
   different types of data content (including potentially mixed types).
   The senders enqueue and transmit bulk content in the form of static
   data or files and/or non-finite, ongoing stream types.  NORM senders
   provide for repair transmission of data and/or FEC content in
   response to NACK messages received from the receiver group.
   Mechanisms for out-of-band information and other transport control
   mechanisms are specified for use by applications to form complete
   reliable multicast solutions for different purposes.

1.3. NORM Scalability

Group communication scalability requirements lead to adaptation of NACK-based protocol schemes when feedback for reliability is needed [RmComparison]. NORM is a protocol centered around the use of selective NACKs to request repairs of missing data. NORM provides for the use of packet-level forward error correction (FEC) techniques for efficient multicast repair and OPTIONAL proactive transmission robustness [RFC3453]. FEC-based repair can be used to greatly reduce the quantity of reliable multicast repair requests and repair transmissions [MdpToolkit] in a NACK-oriented protocol. The principal factor in NORM scalability is the volume of feedback traffic generated by the receiver set to facilitate reliability and congestion control. NORM uses probabilistic suppression of redundant feedback based on exponentially distributed random backoff timers. The performance of this type of suppression relative to other techniques is described in [McastFeedback]. NORM dynamically measures the group's round-trip timing status to set its suppression and other protocol timers. This allows NORM to scale well while maintaining reliable data delivery transport with low latency relative to the network topology over which it is operating. Feedback messages can be either multicast to the group at large or sent via unicast routing to the sender. In the case of unicast feedback, the sender relays the feedback state to the group to facilitate feedback suppression. In typical Internet environments, the NORM protocol will readily scale to group sizes on the order of tens of thousands of receivers. A study of the quantity of feedback for this type of protocol is described in [NormFeedback]. NORM is able to operate with a smaller amount of feedback than a single TCP connection, even with relatively large numbers of receivers. Thus, depending upon the network topology, it is possible for NORM to scale to larger group sizes. With respect to computer resource usage, the
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   NORM protocol does not need state to be kept on all receivers in the
   group.  NORM senders maintain state only for receivers providing
   explicit congestion control feedback.  However, NORM receivers need
   to maintain state for each active sender.  This can constrain the
   number of simultaneous senders in some uses of NORM.

1.4. Environmental Requirements and Considerations

All of the environmental requirements and considerations that apply to the "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401], "Forward Error Correction (FEC) Building Block" [RFC5052], and "TCP-Friendly Multicast Congestion Control (TFMCC) Protocol Specification" [RFC4654] also apply to the NORM protocol. The NORM protocol SHALL be capable of operating in an end-to-end fashion with no assistance from intermediate systems beyond basic IP multicast group management, routing, and forwarding services. While the techniques utilized in NORM are principally applicable to flat, end-to-end IP multicast topologies, they could also be applied in the sub-levels of hierarchical (e.g., tree-based) multicast distribution if so desired. NORM can make use of reciprocal (among senders and receivers) multicast communication under the Any-Source Multicast (ASM) model defined in "Host Extensions for IP Multicasting" [RFC1112], but it SHALL also be capable of scalable operation in asymmetric topologies such as Source-Specific Multicast (SSM) [RFC4607] where only unicast routing service is available from the receivers to the sender(s). NORM is compatible with IPv4 and IPv6. Additionally, NORM can be used with networks employing Network Address Translation (NAT) provided that the NAT device supports IP multicast and/or can cache UDP traffic source port numbers for remapping feedback traffic from receivers to the sender(s).

2. Architecture Definition

A NormSession is comprised of participants (NormNodes) acting as senders and/or receivers. NORM senders transmit data content in the form of NormObjects to the session destination address, and the NORM receivers attempt to reliably receive the transmitted content using negative acknowledgments to request repair. Each NormNode within a NormSession is assumed to have a preselected unique 32-bit identifier (NormNodeId). NormNodes MUST have uniquely assigned identifiers within a single NormSession to distinguish between multiple possible senders and to distinguish feedback information from different receivers. There are two reserved NormNodeId values. A value of 0x00000000 is considered an invalid NormNodeId (NORM_NODE_NONE), and a value of 0xffffffff is a "wild card" NormNodeId (NORM_NODE_ANY).
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   While the protocol does not preclude multiple sender nodes
   concurrently transmitting within the context of a single NORM session
   (i.e., many-to-many operation), any type of interactive coordination
   among NORM senders is assumed to be controlled by the application- or
   higher-protocol layer.  There are some OPTIONAL mechanisms specified
   in this document that can be leveraged for such application-layer

   As previously noted, NORM allows for reliable transmission of three
   different basic types of data content.  The first type is
   NORM_OBJECT_DATA, which is used for static, persistent blocks of data
   content maintained in the sender's application memory storage.  The
   second type is NORM_OBJECT_FILE, which corresponds to data stored in
   the sender's non-volatile file system.  The NORM_OBJECT_DATA and
   NORM_OBJECT_FILE types both represent NormObjects of finite but
   potentially very large size.  The third type of data content is
   NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of
   undefined length.  This is analogous to the reliable stream service
   provided by TCP for unicast data transport.  The format of the stream
   content is application-defined and can be "byte" or "message"
   oriented.  The NORM protocol provides for "flushing" of the stream to
   expedite delivery or possibly enforce application message boundaries.
   NORM protocol implementations MAY offer either (or both) in-order
   delivery of the stream data to the receive application or out-of-
   order (more immediate) delivery of received segments of the stream to
   the receiver application.  In either case, NORM sender and receiver
   implementations provide buffering to facilitate repair of the stream
   as it is transported.

   All NormObjects are logically segmented into FEC coding blocks and
   symbols for transmission by the sender.  In NORM, a FEC encoding
   symbol directly corresponds to the payload of NORM_DATA messages or
   "segment".  Note that when systematic FEC codes are used, the payload
   of NORM_DATA messages sent for the first portion of a FEC encoding
   block are source symbols (actual segments of original user data),
   while the remaining symbols for the block consist of parity symbols
   generated by FEC encoding.  These parity symbols are generally sent
   in response to repair requests, but some number MAY be sent
   proactively at the end of each encoding block to increase the
   robustness of transmission.  When non-systematic FEC codes are used,
   all symbols sent consist of FEC encoding parity content.  In this
   case, the receiver needs to receive a sufficient number of symbols to
   reconstruct (via FEC decoding) the original user data for the given

   Transmitted NormObjects are temporarily, yet uniquely, identified
   within the NormSession context using the given sender's NormNodeId,
   NormInstanceId, and a temporary NormTransportId.  Depending upon the
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   implementation, individual NORM senders can manage their
   NormInstanceIds independently, or a common NormInstanceId could be
   agreed upon for all participating nodes within a session, if needed,
   as a session identifier.  NORM NormTransportId data content
   identifiers are sender-assigned and applicable and valid only during
   a NormObject's actual transport (i.e., for as long as the sender is
   transmitting and providing repair of the indicated NormObject).  For
   a long-lived session, the NormTransportId field can wrap and
   previously used identifiers will be re-used.  Note that globally
   unique identification of transported data content is not provided by
   NORM and, if necessary, is expected to be managed by the NORM
   application.  The individual segments or symbols of the NormObject
   are further identified with FEC payload identifiers that include
   coding block and symbol identifiers.  These are discussed in detail
   later in this document.

2.1. Protocol Operation Overview

A NORM sender primarily generates messages of type NORM_DATA. These messages carry original data segments or FEC symbols and repair segments/symbols for the bulk data/file or stream NormObjects being transferred. By default, redundant FEC symbols are sent only in response to receiver repair requests (NACKs) and thus normally little or no additional transmission overhead is imposed due to FEC encoding. However, the NORM implementation MAY be configured to proactively transmit some amount of redundant FEC symbols along with the original content to potentially enhance performance (e.g., improved delay) at the cost of additional transmission overhead. This configuration is sensible for certain network conditions and can allow for robust, asymmetric multicast (e.g., unidirectional routing, satellite, cable) [FecHybrid] with reduced receiver feedback, or, in some cases, no feedback. A sender message of type NORM_INFO is also defined and is used to carry OPTIONAL out-of-band context information for a given transport object. A single NORM_INFO message can be associated with a NormObject. Because of its atomic nature, missing NORM_INFO messages can be NACKed and repaired with a slightly lower delay process than NORM's general FEC-encoded data content. The NORM_INFO message can serve special purposes for some bulk transfer, reliable multicast applications where receivers join the group mid-stream and need to ascertain contextual information on the current content being transmitted. The NACK process for NORM_INFO will be described later. When the NORM_INFO message type is used, its transmission SHOULD precede transmission of any NORM_DATA message for the associated NormObject. The sender also generates messages of type NORM_CMD to assist in
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   certain protocol operations such as congestion control, end-of-
   transmission flushing, group round-trip time (GRTT) estimation,
   receiver synchronization, and OPTIONAL positive acknowledgment
   requests or application-defined commands.  The transmission of
   NORM_CMD messages from the sender is accomplished by one of three
   different procedures: single, best-effort unreliable transmission of
   the command; repeated redundant transmissions of the command; and
   positively acknowledged commands.  The transmission technique used
   for a given command depends upon the function of the command.
   Several core commands are defined for basic protocol operation.
   Additionally, implementations MAY wish to consider providing the
   OPTIONAL application-defined commands that can take advantage of the
   transmission methodologies available for commands.  This allows for
   application-level session management mechanisms that can make use of
   information available to the underlying NORM protocol engine (e.g.,
   round-trip timing, transmission rate, etc.).  A notable distinction
   between NORM_DATA message and some NORM_CMD message transmissions is
   that typically a receiver will need to allocate resources to manage
   reliable reception when NORM_DATA messages are received.  However,
   some NORM_CMD messages are completely atomic and no specific
   reliability (buffering) state needs to be kept.  Thus, for session
   management or other purposes, it is possible that even participants
   acting principally as data receivers MAY transmit NORM_CMD messages.
   However, it is RECOMMENDED that this is not done within the context
   of the NORM multicast session unless congestion control is addressed.
   For example, many receiver nodes transmitting NORM_CMD messages
   simultaneously can cause congestion for the destination(s).

   All sender transmissions are subject to rate control governed by a
   peak transmission rate set for each participant by the application.
   This can be used to limit the quantity of multicast data transmitted
   by the group.  When NORM's congestion control algorithm is enabled,
   the rate for senders is automatically adjusted.  In some networks, it
   is desirable to establish minimum and maximum bounds for the rate
   adjustment depending upon the application even when dynamic
   congestion control is enabled.  However, in the case of the general
   Internet, congestion control policy SHALL be observed that is
   compatible with coexistent TCP flows.

   NORM receivers generate messages of type NORM_NACK or NORM_ACK in
   response to transmissions of data and commands from a sender.  The
   NORM_NACK messages are generated to request repair of detected data
   transmission losses.  Receivers generally detect losses by tracking
   the sequence of transmission from a sender.  Sequencing information
   is embedded in the transmitted data packets and end-of-transmission
   commands from the sender.  NORM_ACK messages are generated in
   response to certain commands transmitted by the sender.  In the
   general (and most scalable) protocol mode, NORM_ACK messages are sent
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   only in response to congestion control commands from the sender.  The
   feedback volume of these congestion control NORM_ACK messages is
   controlled using the same timer-based probabilistic suppression
   techniques as for NORM_NACK messages to avoid feedback implosion.  In
   order to meet potential application requirements for positive
   acknowledgment from receivers, other NORM_ACK messages are defined
   and are available for use.

2.2. Protocol Building Blocks

The operation of the NORM protocol is based primarily upon the concepts presented in the Multicast NACK Building Block [RFC5401] document. This includes the basic NORM architecture and the data transmission, repair, and feedback strategies discussed in that document. The reliable multicast building block approach, as described in "Reliable Multicast Transport Building Blocks for One- to-Many Bulk-Data Transfer" [RFC3048], is applied in creating the full NORM protocol instantiation. NORM also makes use of the parity- based encoding techniques for repair messaging and added transmission robustness as described in "The Use of Forward Error Correction (FEC) in Reliable Multicast" [RFC3453]. NORM uses the FEC Payload ID as specified by the FEC Building Block document [RFC5052]. Additionally, for congestion control, this document fully specifies a baseline congestion control mechanism (NORM-CC) based on the TCP- Friendly Multicast Congestion Control (TFMCC) scheme [TfmccPaper], [RFC4654].

2.3. Design Trade-Offs

While the various features of NORM provide some measure of general purpose utility, it is important to emphasize the understanding that "no one size fits all" in the reliable multicast transport arena. There are numerous engineering trade-offs involved in reliable multicast transport design and this necessitates an increased awareness of application and network architecture considerations. Performance requirements affecting design can include: group size, heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data ordering, delivery delay, group dynamics, mobility, congestion control, and transport across low-capacity connections. NORM contains various parameters to accommodate many of these differing requirements. The NORM protocol and its mechanisms MAY be applied in multicast applications outside of bulk data transfer, but there is an assumed model of bulk transfer transport service that drives the trade-offs that determine the scalability and performance described in this document. The ability of NORM to provide reliable data delivery is also governed by any buffer constraints of the sender and receiver
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   applications.  NORM protocol implementations SHOULD operate with the
   greatest efficiency and robustness possible within application-
   defined buffer constraints.  Buffer requirements for reliability, as
   always, are a function of the delay-bandwidth product of the network
   topology.  NORM performs best when allowed more buffering resources
   than typical point-to-point transport protocols.  This is because
   NORM feedback suppression is based upon randomly delayed
   transmissions from the receiver set, rather than immediately
   transmitted feedback.  There are definitive trade-offs between buffer
   utilization, group size scalability, and efficiency of performance.
   Large buffer sizes allow the NORM protocol to perform most
   efficiently in large delay-bandwidth topologies and allow for longer
   feedback suppression backoff timeouts.  This yields improved group
   size scalability.  NORM can operate with reduced buffering but at a
   cost of decreased efficiency (lower relative goodput) and reduced
   group size scalability.

3. Conformance Statement

This RMT Protocol Instantiation document, in conjunction with the "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401] and "Forward Error Correction (FEC) Building Block" [RFC5052] Building Blocks, completely specifies a working reliable multicast transport protocol that conforms to the requirements described in RFC 2357. This document specifies the following message types and mechanisms that are REQUIRED in complying NORM protocol implementations: +----------------------+--------------------------------------------+ | Message Type | Purpose | +----------------------+--------------------------------------------+ | NORM_DATA | Sender message for application data | | | transmission. Implementations MUST | | | support at least one of the | | | NORM_OBJECT_DATA, NORM_OBJECT_FILE, or | | | NORM_OBJECT_STREAM delivery services. The | | | use of the NORM FEC Object Transmission | | | Information header extension is OPTIONAL | | | with NORM_DATA messages. | | NORM_CMD(FLUSH) | Sender command to excite receivers for | | | repair requests in lieu of ongoing | | | NORM_DATA transmissions. Note the use of | | | the NORM_CMD(FLUSH) for positive | | | acknowledgment of data receipt is | | | OPTIONAL. |
Top   ToC   RFC5740 - Page 14
   | NORM_CMD(SQUELCH)    | Sender command to advertise its current    |
   |                      | valid repair window in response to invalid |
   |                      | requests for repair.                       |
   | NORM_CMD(REPAIR_ADV) | Sender command to advertise current repair |
   |                      | (and congestion control state) to group    |
   |                      | when unicast feedback messages are         |
   |                      | detected.  Used to control/suppress        |
   |                      | excessive receiver feedback in asymmetric  |
   |                      | multicast topologies.                      |
   | NORM_CMD(CC)         | Sender command used in collection of       |
   |                      | round-trip timing and congestion control   |
   |                      | status from group (this is OPTIONAL if     |
   |                      | alternative congestion control mechanism   |
   |                      | and round-trip timing collection is used). |
   | NORM_NACK            | Receiver message used to request repair of |
   |                      | missing transmitted content.               |
   | NORM_ACK             | Receiver message used to proactively       |
   |                      | provide feedback for congestion control    |
   |                      | purposes.  Also used with the OPTIONAL     |
   |                      | NORM Positive Acknowledgment Process.      |

   This document also describes the following message types and
   associated mechanisms that are OPTIONAL for complying NORM protocol

   | Message Type          | Purpose                                   |
   | NORM_INFO             | Sender message for providing ancillary    |
   |                       | context information associated with NORM  |
   |                       | transport objects.  The use of the NORM   |
   |                       | FEC Object Transmission Information       |
   |                       | header extension is OPTIONAL with         |
   |                       | NORM_INFO messages.                       |
   | NORM_CMD(EOT)         | Sender command to indicate it has reached |
   |                       | end-of-transmission and will no longer    |
   |                       | respond to repair requests.               |
   | NORM_CMD(ACK_REQ)     | Sender command to support                 |
   |                       | application-defined, positively           |
   |                       | acknowledged commands sent outside of the |
   |                       | context of the bulk data content being    |
   |                       | transmitted.  The NORM Positive           |
   |                       | Acknowledgment Procedure associated with  |
   |                       | this message type is OPTIONAL.            |
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   | NORM_CMD(APPLICATION) | Sender command containing                 |
   |                       | application-defined commands sent outside |
   |                       | of the context of the bulk data content   |
   |                       | being transmitted.                        |
   | NORM_REPORT           | Optional message type reserved for        |
   |                       | experimental implementations of the NORM  |
   |                       | protocol.                                 |

4. Message Formats

There are two primary classes of NORM messages (see Section 2.1): sender messages and receiver messages. NORM_CMD, NORM_INFO, and NORM_DATA message types are generated by senders of data content, and NORM_NACK and NORM_ACK messages generated by receivers within a NormSession. Sender messages SHALL be governed by congestion control for Internet use. For session management or other purposes, receivers can also employ NORM_CMD message transmissions. The principal rationale for distinguishing sender and receiver messages is that receivers will typically need to allocate resources to support reliable reception from sender(s) and NORM sender messages are subject to congestion control. NORM receivers MAY employ the NORM_CMD message type for application-defined purposes, but it is RECOMMENDED that congestion control and feedback implosion issues be addressed. Additionally, an auxiliary message type of NORM_REPORT is also provided for experimental purposes. This section describes the message formats used by the NORM protocol. These messages and their fields are referenced in the detailed functional description of the NORM protocol given in Section 5. Individual NORM messages are compatible with the Maximum Transmission Unit (MTU) limitations of encapsulating Internet protocols including IPv4, IPv6, and UDP. The current NORM protocol specification assumes UDP encapsulation and leverages the transport features of UDP. The NORM messages are independent of network addresses and can be used in IPv4 and IPv6 networks.

4.1. NORM Common Message Header and Extensions

There are some common message fields contained in all NORM message types. Additionally, a header extension mechanism is defined to expand the functionality of the NORM protocol without revision to this document. All NORM protocol messages begin with a common header with information fields as follows:
Top   ToC   RFC5740 - Page 16
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version|  type |    hdr_len    |          sequence             |
     |                           source_id                           |

                Figure 1: NORM Common Message Header Format

   The "version" field is a 4-bit value indicating the protocol version
   number.  NORM implementations SHOULD ignore received messages with
   version numbers different from their own.  This number is intended to
   indicate and distinguish upgrades of the protocol that are non-
   interoperable.  The NORM version number for this specification is 1.

   The message "type" field is a 4-bit value indicating the NORM
   protocol message type.  These types are defined as follows:

                  | Message          |       Value      |
                  | NORM_INFO        |         1        |
                  | NORM_DATA        |         2        |
                  | NORM_CMD         |         3        |
                  | NORM_NACK        |         4        |
                  | NORM_ACK         |         5        |
                  | NORM_REPORT      |         6        |

   The 8-bit "hdr_len" field indicates the number of 32-bit words that
   comprise the given message's header portion.  This is used to
   facilitate the addition of header extensions.  The presence of header
   extensions is implied when the "hdr_len" value is greater than the
   base value for the given message "type".

   The "sequence" field is a 16-bit value that is set by the message
   originator.  The "sequence" field serves two separate purposes,
   depending upon the message type:

   1.  NORM senders MUST set the "sequence" field of sender messages
       (NORM_INFO, NORM_DATA, and NORM_CMD) so that receivers can
       monitor the "sequence" value to maintain an estimate of packet
       loss that can be used for congestion control purposes (see
       Section 5.5.2 for a detailed description of NORM Congestion
       Control operation).  A monotonically increasing sequence number
       space MUST be maintained to mark NORM sender messages in this
       way.  Note that this "sequence" number is explicitly NOT used in
Top   ToC   RFC5740 - Page 17
       NORM as part of its reliability procedures.  The NORM object and
       FEC payload identifiers are used to detect missing content for
       reliable transfer purposes.

   2.  NORM receivers SHOULD set the "sequence" field to support
       protection from message replay attacks of NORM_NACK or NORM_NACK
       messages.  Note that, depending upon configuration, NORM feedback
       messages are sent to the session multicast address or the unicast
       address(es) of the active NORM sender(s).  Thus, a separate,
       monotonically increasing sequence number space MUST be maintained
       for each destination address to which the NORM receiver is
       transmitting feedback messages.

   Note that these two separate purposes necessitate the maintenance of
   separate sequence spaces to support the functions described here.
   And, in the case of NORM receivers, additional sequence spaces are
   needed when feedback messages are sent to the sender unicast
   address(es) instead of the session address.

   The "source_id" field is a 32-bit value that uniquely identifies the
   node that sent the message within the context of a single
   NormSession.  This value is termed the NORM node identifier
   (NormNodeId) and unique NormNodeIds MUST be assigned within a single
   NormSession.  In some cases, use of the host IPv4 address or a hash
   of an address can suffice, but alternative methodologies for
   assignment and potential collision resolution of node identifiers
   within a multicast session SHOULD be considered.  For example, the
   techniques for managing the 32-bit "synchronization source" (SSRC)
   identifiers defined in the Real-Time Protocol (RTP) specification
   [RFC3550] are applicable for use with NORM node identifiers when an
   ASM traffic model is observed.  In most deployments of the NORM
   protocol to date, the NormNodeId assignments are administratively
   configured, and this form of NormNodeId assignment is RECOMMENDED for
   most purposes.  NORM sender NormNodeId values MUST be unique within
   an ASM session so that NORM receiver feedback can be properly
   demultiplexed by senders, and NORM receiver NormNodeId values MUST
   also be unique for congestion control operation or when the OPTIONAL
   positive acknowledgment mechanism is used.

   NORM Header Extensions

   When header extensions are applied, they follow the message type's
   base header and precede any payload portion.  There are two formats
   for header extensions, both of which begin with an 8-bit "het"
   (header extension type) field.  One format is provided for variable-
   length extensions with "het" values in the range from 0 through 127.
   The other format is for fixed-length (one 32-bit word) extensions
   with "het" values in the range from 128 through 255.
Top   ToC   RFC5740 - Page 18
   For variable-length extensions, the value of the "hel" (header
   extension length) field is the length of the entire header extension,
   expressed in multiples of 32-bit words.  The "hel" field MUST be
   present for variable-length extensions ("het" between 0 and 127) and
   MUST NOT be present for fixed-length extensions ("het" between 128
   and 255).

   The formats of the variable-length and fixed-length header extensions
   are given, respectively, here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het <=127   |      hel      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                    Header Extension Content                   |
     |                              ...                              |

          Figure 2: NORM Variable-Length Header Extension Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het >=128   |    reserved   |    Header Extension Content   |

       Figure 3: NORM Fixed-Length (32-bit) Header Extension Format

   The "Header Extension Content" portion of the header extension is
   defined for each extension type.  Some header extensions are defined
   within this document for NORM baseline FEC and congestion control

(page 18 continued on part 2)

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